Optical transmission systems, including optical fiber communication systems, have become an attractive alternative for carrying voice and data at high speeds. In optical transmission systems, waveform degradation due to chromatic dispersion in the optical transmission medium can be problematic, particularly as transmission speeds continue to increase.
Chromatic dispersion results from the fact that in transmission media such as glass optical waveguides, the higher the frequency of the optical signal, the greater the refractive index. The refractive index in a bulk material at a particular frequency is defined to be the ratio of the vacuum wavelength of the signal at that frequency to the wavelength of the signal in the material. Equivalently, it is defined to be the ratio of the phase velocity of the signal in the material to the speed of light in vacuum.
In single mode optical fiber, chromatic dispersion can result from the interplay of two underlying effects, material dispersion and waveguide dispersion. Material dispersion results from the non-linear dependence upon wavelength of the refractive index, and the corresponding group velocity of the material, illustratively doped silica. Waveguide dispersion results from the wavelength-dependent relationship of the index of refraction between the core and the cladding and the fraction of the optical intensity contained in each.
Moreover, impurities in the waveguide material, mechanical stress and strain, and temperature effects can also affect the index of refraction, further adding to the ill effects of chromatic dispersion.
In digital optical communications, where the optical signal is ideally a square wave, bit spreading due to chromatic dispersion can be particularly problematic. Because of the spectral dependence of the propagation velocity due to chromatic dispersion, the shape of the waveform can be substantially distorted. The effects of this type of dispersion are a spreading of the original pulse in time, causing it to overflow in the time slot that has already been allotted to another bit. When the overflow becomes excessive, intersymbol interference (ISI) may result. ISI may result in an increase in the bit-error rate to unacceptable levels.
As can be appreciated, compensating for chromatic dispersion is increasingly important in optical communications. One known method for compensating for chromatic dispersion is through the use of specialty fiber known as dispersion compensating fiber (DCF).
Dispersion compensating fibers often have added dopants (e.g., Ge) in the core. While beneficial to the desired end of providing suitable dispersion compensation, adding dopants to the core also increases the effective index of refraction of the core. As a consequence, the fiber can support multiple modes (and thus may be referred to as a ‘few-moded’ fiber). These additional modes can result in multi-path interference (MPI), a well-known system impairment.
MPI results from the existence of a plurality of paths for signal light in an optical network. If light is launched from a single laser source into a fiber, and subsequently divides among these plurality of paths, the light in each separate path will accumulate a certain amount of relative phase delay. If the light from these paths recombines in total or in part, coherently or incoherently, interference can result. This begets intensity variations, and ultimately results in system impairment.
One significant source of MPI is the propagation of multiple transverse modes in the system. Since each mode has a different effective index of refraction, light will experience different path lengths in each mode, resulting in MPI. This is one reason for the desire for single-mode waveguide systems. However, as described above, the need for increasingly accurate dispersion compensation often necessitates the design of waveguides that unfortunately allow the propagation of multiple modes.
From the above description, it is clear that there are certain competing interests in optical communications systems. These include a need to compensate for chromatic dispersion; and a need is to reduce, if not eliminate, MPI.